1. Field of the Invention
The present invention relates to a differential surface acoustic wave filter, and more particularly to a differential surface acoustic wave filter suitable for use for a high frequency filter built in a mobile communication terminal such as a mobile phone.
This application is a counterpart application of Japanese application Serial Number 149112/1999, filed May 28, 1999, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
With progression and realization of high frequency of a mobile communication equipment, development of a surface acoustic wave filter (SAW) has been actively carried out. Most of the conventional surface acoustic wave filters is an unbalanced filter. This prior art unbalanced surface acoustic wave filter is provided with two input terminals and two output terminals and is referred to as a two-terminal-pair surface acoustic wave filter. By connecting one of the two input terminals and one of the two output terminals to a ground potential, the unbalanced surface acoustic wave filter processes an unbalanced signal. Further, in a high frequency circuit such as an amplifier built in the mobile communication equipment, use of the balanced signal rather than the unbalanced signal is suitable for improving the operation characteristic of the high frequency circuit itself, and hence a differential amplification type balanced circuit is often used. Therefore, a balanced/unbalanced conversion circuit, i.e., a balun circuit (balanced/unbalanced) is required between the output terminals of the unbalanced surface acoustic wave filter and the input terminals of the amplifier.
As a prior art balun circuit, there is, for example, one disclosed in xe2x80x9cThe balun Family: MICROWAVE JOURNAL, September 1987; Hu Shuhaoxe2x80x9d (reference 1). The balun circuit disclosed in this reference 1 is constituted by a passive device such as an inductor or a capacitor, a strip line, and others and extensively put into practical use. That is, the balun circuit, as well as the filter, is an important component in the communication equipment, and made into various shapes and used in accordance with the respective frequency bands such as a VHF (Very High Frequency) band, a UHF (Ultra High Frequency) band, an EHF (Extremely High frequency) band, a milli-meter wave band and others. Recently, as the reduction in size, price and weight of the mobile communication terminal such as a mobile phone advances, downsizing and improvement in the characteristic of the balun circuit are demanded.
On the other hand, as a conventional surface acoustic wave filter, there are such filters disclosed in xe2x80x9cA balanced-unbalanced input/output RF-band SAW filter having different input/output impedances; The Institute of Electronics, Information and Communication Engineers, A-11-17, March 1997xe2x80x9d (reference 2), xe2x80x9cJapanese patent application laid-open No. 331232-1997xe2x80x9d (reference 3), xe2x80x9cA New Balanced-Unbalanced Type RF-Band SAW Filter; IEEE MTT-S Digest, September, 1996 (Y. Taguchi, S. Seki, K. Eda)xe2x80x9d (reference 4). With the demand for downsizing or improvement in the characteristic of the balun circuit, reduction in size and improvement in the characteristic of the surface acoustic wave filter are recently requested. Consequently, there is advanced development of a differential surface acoustic wave filter having both the balanced-unbalanced conversion function and the filter function such as the surface acoustic wave filters disclosed in the above-described references 2 to 4.
Here, FIG. 1 shows the structure of a prior art differential surface acoustic wave filter having both the balanced-unbalanced conversion function and the filter function. As shown in FIG. 1, the conventional differential surface acoustic filter 100 is constituted by three interdigital transducers (which will be referred to as IDTs hereinafter) 108, 110 and 112 and two reflectors 114 and 116. One electrode of the input IDT 108 is connected to an input electrode 102 of the differential surface acoustic wave filter 100 to which an unbalanced signal is inputted, and the other electrode of the input IDT is connected to a ground potential. One electrode of each of the output IDTs 110 and 112 is connected to an output terminal 104 of the differential surface acoustic wave filter 100 from which a balanced signal is outputted, and the other electrode of each of the output IDTs 110 and 112 is connected to an output terminal of the differential surface acoustic wave filter 100 from which the balanced signal is outputted. A number of interdigital electrodes of the IDTs 108, 110 and 112 connected to the input terminal 102 and the output terminal 104 is larger than a number of interdigital electrodes of the opposed IDTs 108, 110 and 112. In other words, the input IDT 108 and the output IDTs 110 and 112 are so formed as to face the same direction.
With the above-described arrangement, the differential surface acoustic wave filter 100 which receives the unbalanced signal and outputs the balanced signal is realized. Assuming that a traveling direction of the surface acoustic wave on a piezoelectric substrate is an X axis; a direction parallel to a crystal plane of a monocrystal LiNbO3 and vertical to the X axis, a Y axis; and a normal line direction of the crystal plane of a monocrystal LiNbO3, a Z axis, when the differential acoustic wave filter 100 is formed on the piezoelectric substrate produced by being cut on the plane inclined at 64xc2x0 from the Y axis to the Z axis, the insertion loss of 3 dB and attenuation of 30 dB are obtained as the characteristics of the differential surface acoustic wave filter 100.
As to the above-described balun circuit made up of an inductor, a capacitor and others, realization of downsizing and reduction in the insertion loss are demanded. Additionally, reduction in the insertion loss and increase in the attenuation in a frequency band other than a pass frequency band are desired for the above-described differential surface acoustic wave filter.
It is an object of the present invention to provide a differential surface acoustic wave filter having both an excellent balanced-unbalanced conversion function and a good characteristic without increasing a circuit scale more than necessary.
To achieve this aim, a differential surface acoustic wave filter according to the present invention comprises: a first two-terminal-pair surface acoustic wave resonator having first and second input terminals and first and second output terminals, the first input terminal and the second output terminal being connected to a ground potential; a second two-terminal-pair surface acoustic wave resonator having third and fourth input terminals and third and fourth output terminals, the third input terminal and the fourth output terminals being connected to the ground potential; a fifth input terminal connected to the second and third input terminals, to which an unbalanced signal is inputted; a fifth output terminal connected to the first output terminal of the first surface acoustic wave resonator, from which a first balanced signal having a phase opposite from that of the unbalanced signal is outputted; and a sixth output terminal connected to the fourth output terminal of the second surface acoustic wave resonator, from which a second balanced signal having a phase opposite from that of the unbalanced signal is outputted.
In order to attain this aim, a differential surface acoustic wave filter according to the present invention comprises; a first two-terminal-pair surface acoustic resonator having first and second input terminals and first and second output terminals, the first input terminal and the second output terminal being connected to a ground potential; a second two-terminal-pair surface acoustic wave resonator having third and fourth input terminals and third and fourth output terminals, the third input terminal and the fourth output terminal being connected to the ground potential; a third two-terminal-pair surface acoustic wave resonator having fifth and sixth input terminals and fifth and sixth output terminals, the fifth input terminal being connected to the first output terminal, the sixth input terminal being connected to the second output terminal; a fourth two-terminal-pair surface acoustic wave resonator having seventh and eighth input terminals and seventh and eighth output terminals, the seventh input terminal being connected to the third output terminal, the eighth input terminal being connected to the fourth output terminal; a ninth input terminal connected to the second input terminal in the first two-terminal-pair surface acoustic wave resonator and the third input terminal in the second two-terminal-pair surface acoustic wave resonator, to which an unbalanced signal is inputted; a ninth output terminal connected to the fifth output terminal in the third two-terminal-pair surface acoustic wave resonator, from which a first balanced signal having a phase opposite from that of the unbalanced signal is outputted; and a tenth output terminal connected to the eighth output terminal in the fourth two-terminal-pair surface acoustic wave resonator, from which a second balanced signal having a phase opposite from that of the unbalanced signal is outputted, wherein the sixth output terminal in the third two-terminal-pair surface acoustic wave resonator is connected to the seventh output terminal in the fourth two-terminal-pair surface acoustic wave resonator.